This invention relates to carbides, particularly carbides of group IIa, IIIa, IVa, IVb, Vb, VIb, VIIb and VIIb carbon reactive elements and to their method of preparation.
Carbides may generally be defined as compounds of carbon with metals, transition metals or silicon. Carbides usually have high melting points and are not readily volatilized. They are usually produced by heating appropriate mixtures to high temperatures in electric furnaces.
The largest group of carbides is the acetyledic group, including the carbides of beryllium, calcium, strontium, sodium, potassium, copper, silver, gold, and nickel. The acetylides, the most important of which is calcium carbide, form acetylene by reaction with water or acids. Another group, consisting of aluminum, beryllium, and manganese carbides, is termed the methanides. These yield methane on reaction with water or acids.
Important metallic carbides include iron carbide, or cementite, the hardening constituent in steel; tungsten carbide, from which are made hard tools for the machining of tough metals; and boron carbide, a material almost as hard as diamond. An important nonmetallic carbide is silicon carbide, or carborundum, which is used as an abrasive.
Refractory carbides, i.e. carbides that melt above 1,400xc2x0 C. and are chemically stable are important for the manufacture of high performance materials. Such carbides are usually formed from group IIIa, IVa, IVb, VIb, VIb and VIIIb elements. Examples of refractory carbides and their approximate melting points include: boron carbide 2350xc2x0 C., chromium carbide 1980xc2x0 C., hafnium carbide 3890xc2x0 C., iron carbide 1837xc2x0 C., molybdenum carbide 2692xc2x0 C., niobium carbide 3500xc2x0 C., silicon carbide 2700xc2x0 C., tantalum carbide 3880xc2x0 C., titanium carbide 3140xc2x0 C., tungsten carbide 2870xc2x0 C., vanadium carbide 2810xc2x0 C., and zirconium carbide 3540xc2x0 C. Aluminum carbide 1400xc2x0 C. is also often included within this group even though it may decompose to methane upon exposure to water.
Such refractory carbides have found broad utility due to their high melting points, strength, close crystal structure, electrical properties as insulators or semi-conductors and chemical resistance. Silicon carbide is the most commonly used refractory carbide due to inexpensive materials needed for preparation (silicon and carbon) exceptional hardness, chemical resistance and heat resistance. Silicon carbide is thus most commonly used as an abrasive but has also been used in heat insulating materials, electrical insulating materials, reflective materials and to form heat resistant parts, e.g. in turbines. Boron carbide is one of the hardest materials known to man rivaling the hardness of diamond. Boron carbide may thus be found in high performance abrasives. Iron carbide is used as a hardener in steel and tungsten carbide is used as a hard surface on tools. Germanium carbide has been used in infrared transparent materials and in photoreceptors.
Carbides are generally prepared by four methods including: a) preparation in melt; b) preparation by carburization of powdered metal, metal hydrides or oxides with solid carbon; c) reduction of halides with a hydrogen hydrocarbon gas mixture and d) chemical separation from carbon-saturated ferroalloys or metal baths.
Despite the many advantages of carbide materials, until now the carbides and especially shaped parts from them have been very difficult to manufacture. Most of the chemically resistant refractory carbides are made by reaction of carbon with certain members of the group IIIa, IVa, IVb, Vb, VIb, and VIIIb elements, either as nascent metals or as their oxides according to the equations M+Cxe2x86x92MC or MO+2Cxe2x86x92MC+CO. Such reactions usually occur at very high temperatures in very hot furnaces.
When carbides are made by bulk processing of powdered materials. e.g. electrical resistance heating of a mixture of carbon and silica in a pile around a carbon resistance core, the carbide (silicon carbide) is randomly formed around the core in a large porous structure which permits gas to escape. The silicon carbide must then be separated, in the form of relatively small crystals, from unreacted carbon and silica. Preparation of defined parts by this method is virtually impossible.
Attempts to form carbides into particular shapes at the time of their preparation have met with only limited success. It is difficult to form a specific shape at such temperatures by means of a mold and huge quantities of energy are wasted heating both the mold and furnace. Formed shapes thus often do not meet required tolerances and subsequent machining of such hard materials is difficult and extremely expensive.
It has therefore been found to actually be more cost effective to form carbide materials in bulk, comminute the carbide to micron size powders and then sinter the powders to obtain the desired formed product.